Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session V01: Frontiers of Topological Materials: Mostly Theory |
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Sponsoring Units: DCMP Chair: Daniel Bulmash, University of Maryland, College Park Room: BCEC 106 |
Thursday, March 7, 2019 2:30PM - 2:42PM |
V01.00001: Magnetic quadrupole moment in higher order topological semimetals Jacopo Gliozzi, Mao Lin, Taylor Hughes One characteristic feature of conventional two dimensional Dirac semimetals is the bulk orbital magnetic dipole moment (MDM), which is proportional to the energy separation of the bulk nodes, and manifests as boundary circulating currents. In this presentation, we instead consider the magnetic quadrupole moment (MQM) in the three dimensional higher order topological semimetals (HOTS). In contrast to all semimetals considered previously, the bulk energy bands of a HOTS could be gapped, and its surfaces form one (or half of a) two dimensional Dirac semimetal with surface Dirac nodes. By adding perturbations, an energy difference can be created between pairs of surface nodes. We show analytically and numerically that the nodal energy difference generates a bulk MQM and is associated with circulating hinge currents. We discuss possible experiments in solid state systems as well as in metamaterials to verify our predictions. |
Thursday, March 7, 2019 2:42PM - 2:54PM |
V01.00002: 't Hooft anomalies in symmetry-enriched U(1) gauge theories Shangqiang Ning, Liujun Zou, Meng Cheng The 3+1D U(1) gauge theory is the effective field theory of a three-dimensional U(1) quantum spin liquid, which is an exotic gapless phase with emergent gapless photon and also gapped fractionalized electric charges and magnetic monopoles. In the presence of global symmetries, these fractionalized excitations can transform in different fractionalization patterns, which can be captured by projective representations of the symmetry group. Symmetry properties of these gapped excitations determine the universal properties of a symmetric U(1) quantum spin liquid, as well as the nature of its proximate phases. However, certain patterns of symmetry fractionalization cannot be realized in strictly 3+1D . Instead, they can only be realized on the boundary of 4+1D symmetry-protected topological phases. This type of symmetry fractionalization patterns are referred to as possessing 't Hooft anomalies. We obtain the classification of the patterns of symmetry fractionalization in symmetry-enriched U(1) gauge theories, and, in particular, an explicit formula to determine the relevant 't Hooft anomalies in terms of the fractionalization patterns. |
Thursday, March 7, 2019 2:54PM - 3:06PM |
V01.00003: Type-II Fracton Models with Gapped Boundaries Daniel Bulmash, Arpit Dua, Thomas Iadecola, Dominic Williamson Type-II fracton systems are exotic models whose pointlike excitations are strictly immobile at zero temperature and have locally indistinguishable degenerate ground states, and unusual error correction properties. We study gapped boundaries of such models, including Haah’s code. After enumerating the set of possible gapped boundaries, we study the ground state degeneracy and logical operators in the presence of these boundaries. Although these properties are complicated on the torus, they simplify dramatically for certain open boundary conditions. We discuss the interpretation of the degeneracy in terms of surface line defects, i.e. the domain walls between distinct gapped boundaries. |
Thursday, March 7, 2019 3:06PM - 3:18PM |
V01.00004: Variational Monte Carlo study on a magnetization process of the Kitaev honeycomb model under a magnetic field Kota Ido, Takahiro Misawa Recently, the Kitaev model under a magnetic field has been intensively studied, because the magnetization processes offer useful information for characterizing the nature of the quantum spin liquid. It was reported that an intermediate state appears in the antiferromagnetic coupling Kitaev model (AF KM) under a magnetic field by several numerical methods such as Majorana mean field, exact diagonalization, and DMRG. However, these methods are not appropriate for highly accurate analyses of physical properties in the 2D system with large system sizes. To identify whether the intermediate state exists as the ground state of the AF KM under a magnetic field in the bulk limit, it should be necessary to use an accurate numerical method which is applicable to the large 2D system. |
Thursday, March 7, 2019 3:18PM - 3:30PM |
V01.00005: Model of spin liquids with and without time-reversal symmetry Jyong-Hao Chen, Christopher M Mudry, Claudio Chamon, Alexei Tsvelik We study a model in (2+1)-dimensional spacetime that is realized by an array of chains, each of which realizes relativistic Majorana fields in (1+1)-dimensional spacetime, coupled via current-current interactions. The model is shown to have a lattice realization in an array of two-leg quantum spin-1/2 ladders. We study the model both in the presence and absence of time-reversal symmetry, within a mean-field approximation. We find regimes in coupling space where Abelian and non-Abelian spin liquid phases are stable. In the case when the Hamiltonian is time-reversal symmetric, we find regimes where gapped Abelian and non-Abelian chiral phases appear as a result of spontaneous breaking of time-reversal symmetry. These gapped phases are separated by a discontinuous phase transition. More interestingly, we find a regime where a non-chiral gapless non-Abelian spin liquid is stable. The excitations in this regime are described by relativistic Majorana fields in (2+1)-dimensional spacetime, much as those appearing in the Kitaev honeycomb model, but here emerging in a model of coupled spin ladders that does not break SU(2) spin-rotation symmetry. |
Thursday, March 7, 2019 3:30PM - 3:42PM |
V01.00006: Monolayer Mg2C: Negative Poisson's ratio and unconventional two-dimensional emergent fermions Shan-Shan Wang, Shengyuan Yang Two-dimensional (2D) emergent fermions and negative Poisson's ratio in 2D materials are fascinating subjects of research. Here, we predict that the hexacoordinated Mg2C monolayer hosts both exotic properties. We analyze its phonon spectrum, reveal the Raman-active modes, and show that it has small in-plane stiffness constants. Particularly, the Mg2C monolayer shows an intrinsic negative Poisson's ratio ∼−0.023 along zigzag. The material is metallic at its equilibrium state. A moderate biaxial strain can induce a metal-semimetal-semiconductor phase transition, during which several types of 2D unconventional fermions emerge, including Dirac fermions, the 2D double Weyl fermions in the semimetal phase, and the 2D pseudospin-1 fermions around which three bands cross at a single point on the Fermi level. In addition, uniaxial strains along the high-symmetry directions break the threefold rotational symmetry and reduce the number of Dirac points. Interestingly, it also generates 2D type-II Dirac points. We construct effective models to characterize the properties of these fermions. Our result reveals Mg2C monolayer as an intriguing platform for the study of 2D unconventional fermions, and also suggests its great potential for nanoscale device applications. |
Thursday, March 7, 2019 3:42PM - 3:54PM |
V01.00007: Unconventional Pairing Induced Anomalous Transverse Shift in Andreev Reflection Zhi-Ming Yu, Ying Liu, Yugui Yao, Shengyuan Yang Superconductors with unconventional pairings have been a fascinating subject of research, for which a central issue is to explore effects that can be used to characterize the pairing. The process of Andreev reflection--the reflection of an electron as a hole at a normal-mental-superconductor interface--offers a basic mechanism to probe the pairing. Here we predict that in Andreev reflection from unconventional superconductors, the reflected hole acquires an anomalous spatial shift normal to the plane of incidence, arising from the unconventional pairing. The transverse shift is sensitive to the superconducting gap structure, exhibiting characteristic features for each pairing type, and can be detected as voltage signals. Our work not only unveils a fundamentally new effect with a novel underlying mechanism, but also suggests a possible new technique capable of probing the structure of unconventional pairings. |
Thursday, March 7, 2019 3:54PM - 4:06PM |
V01.00008: Goos-Hänchen-like shifts at a metal/superconductor interface Ying Liu, Zhi-Ming Yu, Hua Jiang, Shengyuan Yang At a normal-metal/superconductor interface, an incident electron from the normal-metal (N) side can be normally reflected as an electron or Andreev reflected as a hole. We show that pronounced lateral shifts along the interface between the incident and the reflected quasiparticles can happen in both reflection processes, which are analogous to the Goos-Hänchen effect in optics. Two concrete model systems are considered. For the simplest model in which the N side is of the two-dimensional electron gas, we find that while the shift in Andreev reflection stays positive, the shift in normal reflection can be made either positive or negative, depending on the excitation energy. For the second model with the N side taken by graphene, the shift in Andreev reflection can also be made negative, and the shifts have rich behavior due to the additional sublattice pseudospin degree of freedom. We show that the shift strongly modifies the dispersion for the confined waveguide modes in an SNS structure. We also suggest a possible experimental setup for detecting the shift. |
Thursday, March 7, 2019 4:06PM - 4:18PM |
V01.00009: Dynamical Localization and Delocalization in Floquet Systems Tilen Cadez, Rubem Mondaini, Pedro D. Sacramento We study the localization aspects of a kicked noninteracting one-dimensional quantum system of spinless fermions and a topological superconductor subject to either time-periodic or aperiodic pulses. The universality class of the transition from delocalized to localized regimes is studied in the case of time-periodic and spatially quasi-periodic kicks. In the case of aperiodic kicks, delocalization ultimately sets in and a diffusive spreading of an initial wave packet is obtained even for small time-aperiodicity of the driving. In the case of Floquet topological superconductors, one finds both Majorana and fermionic localized edge modes in the topological regime. In the intermediate driving period regime, one can identify a region in the phase diagram with a mobility edge between critical and localized states. Finally, we analyze the robustness of the Majorana modes to deviations on the driving period, finding they are stable in a small set of the parameters. |
Thursday, March 7, 2019 4:18PM - 4:30PM |
V01.00010: Prethermalization and topological transport in slowly driven Floquet systems Netanel Lindner, Tobias Gulden, Erez Berg, Mark Rudner Topological phenomena in periodically driven quantum many body systems are difficult to obtain due to the generic tendency of such systems to heat up and tend towards an infinite temperature-like state. We investigate a mechanism to transiently stabilize topological phenomena over a long-time window for systems driven at low frequencies. We derive an analytical bound for the rate of change in the number of particles populating the Floquet bands of the system. The bound is exponentially small in the ratio between the instantaneous bandgap and the maximum between the driving frequency, interaction strength, and the bandwidth. Within the resulting prethermal time window, a quasi-steady state is stabilized, characterized by maximum entropy density subject to the constraint of fixed number of particles in each band. This mechanism provides a route for obtaining long-lived prethermal states with anomalous topological properties, unattainable in equilibrium, such as universal chiral currents in one dimension and magnetoelectric transport in three dimensions. |
Thursday, March 7, 2019 4:30PM - 4:42PM |
V01.00011: Bulk-edge correspondence of periodically driven 2D systems with time-reversal symmetry Xu Liu, Fenner Harper, Rahul Roy We propose an edge invariant for two-dimensional Floquet systems with time-reversal symmetry. This edge index is physically motivated and locally computable. In addition, we generalize an existing construction of a bulk invariant to systems with disorder. Finally, we derive a bulk-edge correspondence which relates the nontrivial bulk behavior with the edge modes present on a boundary at the end of the evolution. Our results provide the first bulk-boundary correspondence for Floquet systems in this symmetry class. |
Thursday, March 7, 2019 4:42PM - 4:54PM |
V01.00012: FLOQUET ENGINEERING OF EXCHANGE INTERACTIONS IN 2D MAGNETIC MATERIALS Swati Chaudhary, Gil Refael, David Hsieh Periodic drive can be used to modify the spin exchange interactions in some magnetic materials. These effects arise mainly due to photon-assisted hopping and can be understood within the framework of periodically driven Fermi-Hubbard model. In real materials, these interactions are usually mediated by the non-magnetic ions, and many different channels are available for the exchange process. We take into account the presence of these ligand ions, and investigate the effects of the periodic drive on magnetic coupling strengths between different neighboring spins in transition metal trichalcogenides for a variety of exchange pathways available in these materials. Additionally, the strength of these interactions depends on the orbitals involved in the superexchange process. We also provide a novel method to control the exchange interactions by manipulating the properties of these orbitals with a periodic drive. In this case, the magnetic coupling strength is altered due to the AC stark shift of the states available for virtual excitations. We discuss two different orbital floquet engineering schemes involving the orbitals of ligand ions and the magnetic ions. |
Thursday, March 7, 2019 4:54PM - 5:06PM |
V01.00013: Light driven magnetic and topological phase transitions in magnetic topological insulator thin films Xiaoyu Liu, Peizhe Tang, Hannes Huebener, Duan Wenhui, Angel Rubio The external manipulation of the magnetism in topological materials would be important both for fundamental interests and practical applications. Herein, we predict that topological and magnetic properties in magnetically doped topological insulator thin films can be well tuned by the laser field. With increasing the strength of the circularly polarized laser (CPL), we can observe a topological phase transition in such system. Remarkably, the CPL could even induce the magnetic phase transition driving thin films from ferromagnetism to paramagnetism. Its critical behavior strongly depends on the quantum quenching, indicating that different Curie temperatures can be observed in different time scales and experimental setups. Furthermore, we propose an all-optical transistor device in which the on-off signal from polar Kerr-rotation angel can be controlled by the applying laser field. Our discoveries not only deepen our understanding of the relationship between topology and magnetism in the magnetic topological insulator in the non-equilibrium regime, but also extend optoelectronic device applications in topological materials. |
Thursday, March 7, 2019 5:06PM - 5:18PM |
V01.00014: Stability of Periodically Driven Topological Phases against Disorder Ramis Movassagh, Oles Shtanko Time-dependent periodic fields are used to create exotic topological phases of matter with potential applications ranging from quantum transport to quantum computing. These nonequilibrium states, at high driving frequencies, exhibit the quintessential robustness against local disorder similar to equilibrium topological phases. However, proving the existence of such topological phases in a general setting is an open problem. We propose a universal effective theory that leverages on modern free probability theory and ideas in random matrices to analytically predict the existence of the topological phase for finite driving frequencies and across a range of disorder. We find that, depending on the strength of disorder, such systems may be topological or trivial and that there is a transition between the two. In particular, the theory predicts the critical point for the transition between the two phases and provides the critical exponents. We compare with exact diagonalizations for driven-disordered 1D Kitaev chain and 2D Bernevig-Hughes-Zhang models and find excellent agreement. This Letter may guide the experimental efforts for exploring topological phases. |
Thursday, March 7, 2019 5:18PM - 5:30PM |
V01.00015: Surface Acoustic Wave Study in Microwave-Induced Non-Equilibrium Electron System Jianli Wang, Chi Zhang, Loren Pfeiffer, Kenneth West, K. W. Baldwin Microwave-induced resistance oscillations (MIRO) and zero-resistance states (ZRS) occur under microwave irradiation in the ultrahigh mobility two-dimensional electron gas (2DEG) in GaAs/AlGaAs quantum wells (QW). We employed the surface acoustic wave (SAW) to study the structure of MIRO and ZRS. We observe that the velocity shift (Δv/v) shows minima at the peak resistance regime (j ≡ ω/ω_{C }~ 1, 2, 3, etc). And in the ZRS regime, an increase of the velocity shift appears at j = 5/4. The results conform to the theoretical and experimental studies of current domains at ZRS under high power microwave. |
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